The invention generally relates to electromagnetic radiation shielding construction for radio frequency circuitry. More particularly, the invention relates to a means for effectively ground coupling a detachable cover plate for the shield housing of high frequency radio circuitry.
Properly shielding electronic components from electromagnetic radiation has always been a problem in communications circuitry. Providing adequate shielding in MHz and GHz frequencies communications systems with the methods currently used has not proved reliable in light of the advent of circuit miniaturization through integrated circuitry. The shielding is provided by a conductive metal enclosure which is held at a fixed electrical potential. Ordinarily, the enclosure has two portions, the first portion defines the housing for the electronic circuitry. The housing completely encloses the circuitry except for an access opening. The second portion of the enclosure is the cover plate which closes the access opening and thus completely surrounds the circuitry with a electromagnetic shield. In low frequency applications it is only necessary that electrical contact between the cover plate and the housing be made sporadically when the two enclosure pieces are mated. The points of electrical contact along the joint of the cover and housing need only be less than the one-twentieth wavelength of low frequency RF radiaton. At high radio frequencies, (i.e. MHz and GHz) the associated one-twentieth wavelength of an electromagnetic wave becomes sufficiently short so that even a small gap in the electrical and physical connection of two separate pieces of housing can be enough to allow a leak of the high frequency electromagnetic radiation through the shield. At lower RF frequencies, this is not a substantial problem since the one-twentieth wavelengths of the lower RF frequencies are of a physical length such that gaps between good electrical contact points where the two housing parts join are rarely sufficiently large enough to let the relatively long one-twentieth wavelength of low frequency RF to leak through the shield.
One prior art means for connecting two pieces of an enclosure to provide effective low frequency RF shielding involves utilization of the resilient qualities of one of the two enclosure pieces. Usually the first enclosure piece, or housing, is formed of aluminum in a casting process. The second enclosure piece, or access hole cover plate, is preferably stamped out of cold rolled steel. The steel has a characteristic resiliency which is advantageously used to provide a good electrical connection between the two enclosure pieces.
Aluminum is a good conductor which provides a high quality electromagnetic shield when cast into a housing mold. Unfortunately the aluminum surface almost immediately begins to oxidize with the available oxygen in the atmosphere of the housing's environment. This oxidization results in a insulating skin covering the surface of the aluminum housing. The cover plate which covers the access hole in the housing and completes the electromagnetic shield enclosure for the RF circuitry enclosed must have a means for mating with the housing that effectively cuts through the aluminum surface insulating oxide layer. Use of the force resulting from the yield resistance of a resilient material like cold rolled steel has been effective for low frequency RF frequencies. But at high RF frequencies the force from the yield resistance of most materials is not sufficient to give consistent good electrical contact through the oxide layer at points sufficiently close along the housing and cover plate border so as to effectively stop the leakage of the much shorter one-twentieth wavelength radiation of high frequency RF.
Ordinarily, the perimeter of the access hole cover plate includes flange segments which function to attempt sufficient electrical connection through the aluminum housing oxide layer to form a good RF gasket. The flange segments are shaped to produce a force against the mating housing portion of the shielding enclosure in response to the flange segments compression against a part of the housing surface. The line of contact formed by the cold rolled steel flange segments engagement with the interior wall of the cast aluminum housing is not always consistent in its contact and is subject to poor conductivity caused by the flange segment's sporadic ability to cut through the oxide layer on the surface of the cast aluminum.
The flange segments are often cut into multiple finger-like projections. By design, each finger-like projection is hopefully assured to contact the housing in at least one point since each finger-like projection is located on the cover plate such that it must yield against the housing cavity side wall when the two enclosure portions are joined. The number of finger-like projections on each flange segment is directly related to the required closeness of electrical contact points along the area where the housing and cover join. As explained earlier, with lower RF frequencies the gap between cover plate and housing can be relatively large without risking significant radiation leakage. As the frequencies get higher and the wavelengths shorten, the tolerable gap between the points of conduction along the line of contact between the housing and cover plate becomes smaller and smaller. In order to assure the gap lengths remain within acceptable limits the flange segment must be sectioned into more numerous and more closely packed finger-like projections. But as the finger-like projections are made more numerous, they individually are made more narrow. Accordingly, their yield strength decreases, which provides the force by which the finger-like projections press against the housing cavity side wall. Thus their ability to cut through the insulating oxide layer on the aluminum surface is impeded.
Attempts could be made to modify the flange segments and their finger-like projections so that they will provide sufficient force to effectively scrape away the oxide layer at their point of contact with the housing inside wall and thus guarantee adequately closely spaced grounding connections for good shielding at high frequency RF operation. The force by which the finger-like projections press against the inside housing wall is determined by the yield strength of the fingers. One way to increase the yield strength of the finger-like projections would be to increase the thickness of the cover plate. Increasing the thickness of the flange finger-like projections creates manufacturing problems since the thickness of the projections approach the width dimension of the projection. To die stamp out thick and narrow projections from cold rolled steel gives poor results and short die life. Also the force required to cause the thicker projections to yield and allow the cover to fit over the access opening of the housing would increase to a point which the force could no longer be feasibly created by hand pressure alone.
A second prior art means for interconnecting two pieces of an enclosure to provide effective low frequency RF shielding involves the utilization of a conductive non-corrosive mesh, preferrably stainless steel, fitted into a channel formed along the planar shoulder of an access opening in the housing portion of the enclosure. The depth of the channel is not sufficient to totally contain the conductive mesh. Therefore when a flat planar cover plate is placed over the housing access opening, the conductive mesh is compressed into the channel. The purpose of compressing the mesh is to assure good electrical contact between the housing portion of the enclosure and the cover plate of the enclosure.
As with the first mentioned prior art means for interconnection, this compressed mesh method for providing an electrical connection between a housing and a cover plate has characteristics which make its performance unsatisfactory at high RF frequencies. The cover plate is usually a planar sheet of conductive metal, preferably steel. The compressed mesh produces a force against the wall of a cast aluminum housing which will not consistently pierce the oxide layer on the aluminum at points close enough to guarantee the mesh to be a good electromagnetic gasket at high RF frequencies.
Therefore, it is a object of this invention to provide an electrical connection means for two enclosure sections for use as an electromagnetic shield in high frequency RF applications.
It is a further objective of the invention to provide a cover plate for a grounded cast aluminum housing which assures good electrical connection between the housing and the cover plate along the line where the two meet when joined so as to provide an effective electromagnetic shield.